Those skilled in the art of non-cryogenic gas separation continue to search for improved adsorbents which demonstrate higher capacity and selectivity. Molecular sieves such as crystalline microporous aluminosilicates possess cationic sites which are capable of physically adsorbing a more strongly absorbable component of a gaseous mixture containing a more absorbable component and one or more less absorbable components. The gaseous mixture becomes depleted in the more absorbable component providing a gaseous mixture which is enriched in the less strongly absorbable components. A typical example of a gaseous mixture is air wherein the more absorbable component is typically nitrogen and one of the less absorbable components is oxygen.
Molecular sieves are broadly defined as materials which contain porosity at the molecular level and which adsorb differently sized molecules at different rates. Molecular sieves have a broad range of elemental compositions. One type of molecular sieve consists of metallosilicates which have a variety of structures. Depending on the elemental composition these metallosilicates can have a variety of empirical formulas but all consist of an extended crystalline lattice made up of tetrahedrally arranged silicate species forming an open framework structure. The majority of metallosilicates which behave as molecular sieves possess extra framework cations to balance the framework charge.
The most common example of metallosilicate molecular sieves are zeolites. Zeolites are aluminosilicates which can have a variety of Si/Al ratios in their crystalline composition. Both natural and synthetic zeolites are known and may generally be described by the following general formula: EQU M.sub.2/n O:Al.sub.2 O.sub.3 :YSiO.sub.2 :ZH.sub.2 O
where M is a cation, n is its valence, Y is the moles of silica, and Z is the moles of water in the hydrated form. When the water of hydration is removed from the zeolite, highly porous materials are formed which have high internal surface area. Such zeolites can be used as catalysts, supports for metal-containing catalysts and as adsorbents. Zeolites in their hydrated form readily exchange their extra framework cations for other cations producing new cation forms of the zeolite. For this reason, zeolites can also be used as ion exchangers or detergent builders.
Crystalline aluminosilicates can also undergo isomorphous substitution with other elements expanding the range of compositions. Boron and Gallium have been substituted for aluminum in some zeolites. The degree of isomorphous substitution and the specific elements incorporated must be developed specifically for each type of zeolite. High degrees of isomorphous substitution in aluminum-rich zeolites has not been accomplished.
Aluminosilicate molecular sieves having high cation charge density such as calcium or lithium are known adsorbents suitable for use in processes for separating gaseous mixtures. The cation densities possible in aluminosilicates limit the adsorptive gas capacities at ambient and higher temperatures. This limitation occurs when the crystalline framework of such materials become saturated with aluminum. Aluminum saturation in crystalline aluminosilicates typically occurs when the mole ratio of silicon to aluminum is 1:1. This aluminum-rich composition in X-type zeolites is referred to as low silica X (LSX).
Those skilled in the art of molecular sieves are searching for ways to increase the framework charge beyond that possible in aluminosilicates so that higher cation densities are possible. The presence of such additional cations associated with open framework metallosilicates could increase gas capacity of the adsorbent for use in equilibrium-based gas separation applications.
EP-A-0476901 teaches a zeolite having a free aperature main channel of a size greater than 2.2 .ANG., a Si/Al molar ratio of less than 124/1 and zinc in the framework structure. Suitable zeolites are prepared by reacting together at least a source of silicon, a source of aluminum, a source of zinc, a source of alkali metal ions and a source of hydroxyl ions. Alternatively, suitable zeolites may be prepared by reacting a zeolite having a main channel of a size greater than 2.2 .ANG. and a Si/Al molar ratio of less than 124/1 with a compound capable of removing aluminum or silicon from the framework structure and providing zinc ions for introduction into the framework structure in place of the aluminum or silicon.
EP-A-0476901 further states that the zeolites may be further modified by ion exchange with cations. When the ions to be introduced are zinc, such zinc ions are in an octahedrally coordinated form. The examples present zeolites which contain less than one zinc atom per unit cell (i.e. a molar ratio of Zn/Si.ltoreq.0.008) and have less cations than required for charge compensation (1 for every Al and 2 for every Zn). Thus, one cannot assess whether zinc atoms have actually been incorporated into the framework of the zeolites described in the Specification.
Cruceanu and coworkers (Al.I Cuza lasi, Sect. 1 c, 1972 18(2) 223-9) teach that zinc atoms may be incorporated into a zeolite outside the phase fields of either X or A zeolite. Such zeolites are prepared by using zinc acetate in a gel preparation process. The reference does not provide elemental compositions of the products and the adsorptive capacities are essentially the same for adsorbents which contain zinc and adsorbents which do not contain zinc.
U.S. Pat. No. 5,070,052 teaches compositions comprising a zeolite and zinc or a zinc plus an alkali metal and/or an alkaline earth metal compound wherein the sum of the amount of the zinc or zinc plus alkali metal and/or alkaline earth metal in the compound plus any metal cation exchanged in the zeolite is in excess of that required to provide a fully metal cation-exchanged zeolite. The catalyst is preferably activated before use as a catalyst by heating to 400.degree. C. to 650.degree. C., typically in a nitrogen or air atmosphere.
Methods are known for preparing aluminum-rich zeolites. For example, U.S. Pat. No. 5,366,720 teaches that low silica forms of faujasite-type zeolites can be prepared from more siliceous forms of the same zeolite species by contacting the starting zeolite with a highly caustic concentrated sodium aluminate solution at elevated temperatures. The process can be used to produce forms of zeolite X having SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios below 2.5.